Apparatus and method for generating a layer system

ABSTRACT

For improving the variability in the coating of substrates a coating apparatus is proposed having a plasma generator for generating a plasma jet which exits from a coating head of the plasma generator. A first particle reservoir and a second particle reservoir are provided. The particles from the first particle reservoir and the second particle reservoir are supplied to the plasma jet as a particle mixture via a transport pipe. A supply control device is provided for setting the amount of particles from the first particle reservoir fed into the transport pipe relative to the amount of particles from the second particle reservoir.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from German Patent Application No. 10 2012 108 919.1, filed on Sep. 21, 2012, which application is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The invention relates to a coating apparatus for coating a substrate with a plasma generator.

Furthermore the invention relates to a method for coating a.

BACKGROUND OF THE INVENTION

The complex requirements of modern engineering and material challenges implies, to a growing extent, the use of material combinations, amongst them compound materials and layered systems. Such layered systems may for example be used as protective or functional layers on objects against corrosive, thermal, chemical, or biological stresses in many ways. For making such material or layer compounds currently various technologies are employed. Therein often chemical vapor deposition (CVD) or physical vapor deposition (PVD) are used, Further established methods are soldering, diffusion welding, or powder metallurgical compound pressing with possible subsequent smithing. Therein the layers are either applied onto a compact substrate via the melted phase (thermal spraying) or via the vapor (PVD) or gas (CVD) phase, or are directly connected with a compact substrate material as compact parts by means of an auxiliary substance (soldering) or by simultaneous application of pressure and temperature (diffusion welding).

These known techniques have method specific limitations, however. Unfavorable layer properties like for example open porosity and cracks in the layer reduce the protective effect against reactive media. Due to temperature gradients between the materials during production of the layer compounds often stresses remain in the thermally affected regions of the parts. Therefore often laborious additional processes are required.

These disadvantages can often be reduced or completely eliminated by the direct application of layers by means of a plasma jet to which powder is supplied. Such a method for example is known from U.S. Pat. No. 5,853,815. In this document it is proposed to homogeneously coat a substrate with a plasma stream covering the entire width of the substrate. A particle reservoir is directly connected with a plasma generator via a pipe. A large pressure difference between the plasma gun and the plasma generator creates a shock pattern, causing the coating stream to fan out widely and also resulting in a thorough distribution of the coating material in the plasma stream.

Various material combinations can be applied onto a substrate in this way. To this end for example a powder consisting of a mixture of several types of material is used. In this way many material combinations can be applied even on substrates of complex shape, given a corresponding control of the nozzles. For example, in this way a very wear resistant but brittle material can be embedded into an elastic matrix. It is furthermore possible to sinter powders comprising a mixture of plural fine grained metallic components during the coating process.

From DE 199 58 473 A1 a method and an apparatus are known wherein by means of a plasma a multilayered structure is applied onto a substrate. Therein the properties of the individual layers can be chosen from within a wide range. To this end it is proposed to supply to the plasma jet exiting from the plasma generator the species forming the layer, so called precursor materials, in the form of powder, gases, or liquids, which then are chemically or physically changed in the plasma in such a way that they are deposited as a cluster in the nano or microscale range on the substrate. In this way a composite layer system can be applied if precursor materials with different properties are supplied to the plasma jet at different locations. A disadvantage of this method of applying layers to substrates is that the property of the layer to be applied is fixed in the process.

The international application PCT/DE2006/000638, published as WO 2006/108395 A1 describes an apparatus and a method for plasma coating. A plasma generator with plural expansion stages is disclosed, wherein each expansion stage exhibits an inlet for a coating material. Downstream from the expansion stages a mixing chamber is provided, in which the coating materials are mixed with each other and with the plasma.

The German patent document DE 10 2008 053 640 B3 discloses a coating method in which a layer is sprayed onto an object. The spray material is melted from wires by an electric arc. A filler material can be supplied to the spray jet via an injector.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an apparatus for coating a substrate, wherein the properties of the coating to be applied are changeable during the coating process.

According to the invention this object is achieved by a coating apparatus tier coating a substrate, comprising

-   -   a plasma generator for generating a plasma jet; which exits from     -   a coating head of the plasma generator from which the plasma jet         exits;     -   a first particle reservoir connected with a transport pipe for         supplying particles stored in the first particle reservoir to         the plasma jet:     -   at least a second particle reservoir is provided and configured         to supply particles from the second reservoir via the transport         pipe to the plasma jet in a particle mixture with the particles         from the first particle reservoir; and     -   a supply control device for setting an amount of the particles         fed from the first particle reservoir into the transport pipe         relative to the amount of the particles fed from the second         particle reservoir into the transport pipe.

A further object of the invention is to provide a method by which the possibilities of coating substrates become more varied.

With respect to the method the object is achieved by a method for coating a substrate comprising the following steps:

-   -   generating a plasma jet with a plasma generator having at least         one coating head from which the plasma jet exits;     -   feeding particles from at least a first particle reservoir and         from at least a second particle reservoir via a transport pipe         to a supply control device in which they are mixed;     -   supplying a particle mixture of particles from the first         particle reservoir and of particles from the second particle         reservoir from the supply control device to the plasma jet via         the transport pipe; and     -   directing the plasma jet together with the particle mixture onto         a surface of the substrate in order to form the coating.

A coating apparatus for coating a substrate is proposed. The coating apparatus comprises a plasma generator for generating a plasma jet, wherein the plasma jet exits from a coating head of the plasma generator. Particles from a first particle reservoir can be supplied to the plasma jet via a transport pipe. A second particle reservoir is provided from which particles can also be supplied via the transport pipe to the plasma jet. A supply control device in the transport pipe allows setting the amount of particles from the first particle reservoir relative to the amount of particles from the second particle reservoir. Advantageously this ratio of amounts of particles can be varied even during the coating process. This also makes possible the generation of a changing layer profile on the surface of the substrate.

In a preferred embodiment of the coating apparatus a controller for controlling the amount of particle mixture supplied to the plasma jet is provided. Therein the controller may be configured in such a way that the amount of supplied particles can be varied over a wide range, even during the coating process. Moreover, the controller may be a switch or configured to exhibit a switching function so that by this switch the supply of particles to the plasma jet may be allowed or interrupted.

In a further embodiment of the invention a plurality of particle reservoirs is provided. The particle reservoirs therein may be mixed with respect to their relative amounts by a common supply control device or may be applied onto the surface of the substrate with corresponding separate coating heads.

Preferentially for each particle reservoir there is provided at least one separate process by which a fluidized powder is generated from the particle reservoirs. The particle reservoir and the corresponding process gas form a respective particle supply unit. The particle supply unit may comprise a process gas control unit for controlling the mixing relation between the particles and the process gas.

In a further embodiment of the invention the coating apparatus may comprise at least a second coating head and a further particle supply unit corresponding to the second coating head. The particle supply unit therein exhibits a further particle reservoir, a process gas, and a process gas control unit. With this embodiment of the invention it is also possible to provide a plurality of coating heads and respectively corresponding particle supply units,

In the method for coating a substrate in a first embodiment the coating is done with a coating apparatus having a plasma generator for generating a plasma jet and also having a coating head, from which the plasma jet exits. For coating the substrate particles are supplied to the plasma jet from a first particle reservoir via a transport pipe. Also, particles from a second particle reservoir are mixed with those from the first particle reservoir by a supply control device and then fed into the transport pipe together and supplied to the plasma jet as a particle mixture. The plasma jet, together with the particle mixture, is then directed onto the surface of the substrate for forming the coating. Therein the particles from the first particle reservoir may be fluidized with a first process gas and the particles from the second particle reservoir may be fluidized with a second process gas. The fraction of particles from the first particle reservoir within the mixture can be set between 10% and 90%, and the fraction of particles from the second particle reservoir can be set between 10% and 90%. Furthermore it is possible to vary the amount of particles from the first particle reservoir relative to the amount of particles from the second particle reservoir during the coating of the substrate by changing the mixing ratio between the first and second particles during the application.

In a further embodiment of the method according to the invention the coating is done with a coating apparatus having a plasma generator for generating a plasma jet and also having a coating head from which the plasma jet exits. Therein the substrate is coated by supplying particles from a first particle reservoir via a transport pipe to the plasma jet at a first supply location and supplying particles from a second particle reservoir to the plasma jet at a second supply location in such a way that on the substrate a first layer of particles from the first particle reservoir and a second layer of particles from the second particle reservoir are formed. As an alternative, the first and second supply location may also be chosen in such a way that a gradient layer or a compound layer is formed on the substrate.

The second layer or gradient layer or compound layer in a further embodiment of this method is covered with a further layer, wherein particles from a third particle reservoir are fed into a further transport pipe, then are supplied to the second plasma jet of a second coating head, and then are applied onto the second layer of particles from the second particle reservoir or onto the gradient layer or onto the compound layer.

With the method according to the invention and the apparatus according to the invention the properties of the layer to be applied may be varied over a wide range. By specific controlled supply of coating materials into the plasma coating process functional compound layers may be applied. The thickness and the composition of the compound layer therein may be controlled in such a way that the desired electrical, mechanical and chemical properties can be tailored. Also plural layers, including with different properties, and gradient layers may be generated on the substrate.

According to an additional object of the invention a coating apparatus for coating a substrate is provided. The coating apparatus has at least a first plasma generator and at least a second plasma generator each of which generating a plasma jet. The first plasma generator has a coating head from which the plasma jet exits. The second plasma generator has a coating head from which the plasma jet exits. A first particle reservoir is connected with a transport pipe for supplying particles stored in the first particle reservoir to the plasma jet of the first plasma generator. At least a second particle reservoir is provided and configured to supply particles from the second reservoir via the transport pipe to the plasma jet of the first plasma generator in a particle mixture with the particles from the first particle reservoir. A least a third particle reservoir is connected with a transport pipe for supplying particles stored in the third particle reservoir to the plasma jet of the second plasma generator. A supply control device is provided for setting an amount of the particles fed from the first particle reservoir into the transport pipe to a first injector relative to the amount of the particles fed from the second particle reservoir into the transport pipe to a second injector, wherein the first injector and the second injector are arranged in relation to the plasma jet of the first plasma generator. A further supply control device is provided for setting an amount of the particles fed from the third particle reservoir into the transport pipe to a third injector arranged in relation to the plasma jet of the second plasma generator.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional advantages and advantageous embodiments are presented in the subsequent figures and pertaining description, where

FIG. 1 is a schematic coating apparatus with a plasma module for providing a plasma jet;

FIG. 2 is a schematic further embodiment of a coating apparatus with two plasma modules, wherein each of which provides a plasma jest;

FIG. 3 a through c are examples of layers that may be formed with the coating apparatus, in schematic representation;

FIG. 4 is a schematic representation of a possible layered structure on a substrate after a coating;

FIG. 5 is a schematic representation of the principle of a gradient layer by a depth profile; and

FIG. 6 is a schematic representation of an example of a conductive coating formed with the coating apparatus.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 schematically shows a coating apparatus 10 for coating a substrate The coating apparatus 10 has a plasma module with a coating head 26, a source for a plasma process gas 56 and a power supply 58.

The coating head 26 has a plasma chamber 60 in which an electric arc 20 is started between two electrodes 62 and 64. Electrical energy is supplied to this electric arc 20 from the power supply 58 for sustaining it, so that, depending on the modulation of the power supply 58, a continuous plasma jet 22 or a pulsed plasma jet 22 is generated, which exits on the exit side 26A of the coating head 26. At the feed side 26E of the coating head 26 a plasma process gas 56 may be supplied, so that the plasma process gas 56 streams through the plasma chamber 60 in a controlled manner. A mixture of process gas 30, 32 and particles may be supplied to the plasma jet 22 via an injector 66, which here is shown as an external injector. The particles may be partially molten by the high energy density in the plasma jet 22. In this way they can be deposited on the surface 12 a of the substrate 12 as first layer 50. As the substrate 12 and the coating head 26 are moveable relative to each other, a continuous layer 50 can be formed on the substrate 12.

The particle mixture supplied to the injector 66 in the embodiment of the invention shown in FIG. 1 is provided by a first particle supply unit 34 and a second particle supply unit 36. A process gas control unit 38, 42 is provided in the particle supply units 34, 36, respectively. By the process gas control unit the fractions of particles in the respective process gas 30, 32 can be controlled independently of each other. if necessary different process gases 30, 32 may be used in each particle supply unit 34, 36, the process gases being adapted to the particles in the particle reservoirs. From the mixture of particles and process gases 30, 32 fluids are generated, which can be mixed in varying amounts relative to each other by a supply control device 18. The mixture depends on the layer 50 desired on the substrate 12. Usually the mixing ratio of the particles is chosen such that the fraction of the particle mixture with particles from the first particle reservoir 14 is set between 10% and 90%, and that the fraction of particles from the second particle reservoir 16 is set between 10% and 90%.

The supply control device 18 therein is configured such that a ratio which is constant in time between the amount of particles from the first particle reservoir 14 and the amount of particles from the second particle reservoir 16 can be set for the particle mixture. Furthermore also supply control devices 18 may be employed by which in addition or exclusively a time-varying mixing relation can be set. During the supply of particles it is also possible, at least temporarily, to set the relative amount of one of the particle types to 0, so that for a specific part of the surface of the substrate 12 the applied first layer 50 contains only particles from one particle reservoir.

The supply control device 18 may for example be media adder. Therein two fluids may be supplied as two or more partial streams to one or more mixing chambers within the media adder, in which the mixing occurs. The mixing reaction may be controlled, wherein also a time-varying mixing ratio can be set, The mixture is then usually released through an opening in the bottom or top of the mixing chamber and supplied to the transport pipe 24, which for example may be a system of hoses. For the transport pipe 24 also materials different from hoses can be used, like for example metal pipes, depending on the particles which are to be used for coating the substrate 12. Via the transport pipe 24 the particle mixture reaches the injector 66. Upstream of the injector 66 a controller 28 may be provided, by which the amount of particle mixture supplied to the injector 66 is controlled. Control may include a throttling of the particle stream or a dynamical switching process, i.e. controlled blocking and opening of the path to the transport pipe 24 in the controller 28.

With this apparatus dynamically changeable layers 50 may be applied. Thickness and material composition can be dynamically set via the supply rates of the particle supply units 34, 36 and the controller 28. In this way the composition of a layer may also be dynamically changed during an active coating process.

FIG. 2 schematically shows a further embodiment of the apparatus for coating a substrate 12. According to this embodiment of the invention plural, in the example shown two, injectors 66, 68 correspond to the coating head 26. Again the particles from the particle supply units 34, 36 are fluidized in the desired fractions. Afterwards the particles from the particle supply unit 34 are separately supplied to a first injector 66 and enter the plasma jet 22 at a first supply location 46. The particles from the particle supply unit 36 are supplied to a second injector 68 and enter the plasma jet 22 at a second supply location 48. Upstream from the injectors 66, 68 respective supply control devices 18 may be provided, the action of which has already been described in the context of FIG. 1. Through this arrangement two separate layers 50, 52 (double layer), independent of each other, can be generated on the surface 12 a of the substrate 12, the properties of which may be different (see FIG. 6).

There is also the possibility to form a so called gradient layer 54 (see FIG. 3 c) with this apparatus. This is particularly advantageous, as both the double layer and the gradient layer 54 can be applied onto the substrate 12 in one process step. Depending on the arrangement of the injectors 66, 68 and therefore depending on the position of the supply locations 46, 48 relative to the plasma jet 22 a wide range of effects can be achieved. These depend on the injection taking place in different regions of the plasma jet 22. These regions differ by jet velocity, temperature, and plasma composition. Depending on the fluid dynamical mixing of the material streams, multi layers or mixed layers result (FIG. 3).

In FIG. 2 there is furthermore schematically shown that the process carried out with the coating head 26 can be extended, To this end a further coating head 27 can be added to the coating apparatus 10. In the simplest case a plasma process gas 56 and a power supply 58 are provided for this coating head 27 on its feed side 27E. Furthermore there corresponds to it a third particle supply unit 37, which in turn has a particle reservoir 15 and a process gas 33. With the process gas control unit 44 the ratio of process gas 33 and particles from the particle reservoir 15 can be set. By means of an already described supply control device 18 the amount of particles from the particle reservoir 15 can be controlled. Thus a third layer 53 can be deposited onto the second layer 52.

In order to form a layer system with more than three layers or a layer system with two or more gradient layers, the coating apparatus 10 may be provided with a further coating head 26 and two injectors 66, 68, which correspond to the one described above, instead of the simple coating head 27 described.

FIG. 3 a schematically shows a layered structure which may be formed with a coating apparatus 10 according to FIG. 2. Therein a first layer 50, a second layer 52, and a third layer 53 have been applied onto the substrate 12.

FIG. 3 b schematically shows a so called compound layer 55, which may be formed with a coating apparatus 10 according to FIG. 1 or 2. Therein the particles from the particle reservoirs 14, 16 are mixed by a mixing process (FIG. 1) or by an adequate choice of the supply locations 46, 48 in such a way that an as homogeneous as possible distribution of the particle types within the volume of the applied compound layer 55 results.

FIG. 3 c schematically shows a gradient layer 54 which can be formed with the coating apparatus 10 according to FIG. 2. Therein the supply locations 46, 48 are chosen in such a way that the amount of particles in y-direction decreases or increases, respectively.

FIG. 4 schematically shows that it is possible to create various transitions in the sequence of layers to be applied onto the substrate 12. To this end the shown sequence of layers is formed during a single coating run through a suitable configuration of the coating apparatus 10.

In segment A three different materials with the particles r, s, t are deposited with a fixed ratio onto the substrate as a layer. In segment B, later in time during the same coating process, the layer thickness of the compound layer 55 is reduced continuously, and a cover layer of phase u applied on the compound layer 55. in segment C the layer thickness of the entire multilayer is reduced, until in segment D the layer is interrupted completely and thus the substrate 12 is not covered by a layer at this location. In segment E the layer thickness of the phase u is increased continuously and in regions F transitions into a gradient layer 54, in which at the surface of the phase u the material r is embedded at the highest concentration.

FIG. 5 schematically shows the principle of the design of a gradient layer by means of a depth profile. The material composition starts from a layer material S1 having the highest concentration at the transition point to the substrate 12. Towards the surface the layer material S1 decreases continuously, reaching essentially the value zero at the surface. The layer material S2 essentially has the value 0 at the transition point to the substrate 12 and continuously increases towards the surface. In the example shown there is a transition region U, in which the layer material S1 and the layer material S2 have an essentially equal concentration.

FIG. 6 shows a particular application of the coating apparatus 10 according to the invention and the method according to the invention for coating a substrate 12 with the example of a conductive layer 74 and an insulating layer 72. Both layers are applied onto a substrate 12 with the coating apparatus 10. Therein the conductive layer 74 is applied onto the substrate 12 as a strip-like structure. The conductive strip formed this way is to be protected towards the outside by an insulating layer 72 in the region KO. Therein the insulating layer may be interrupted in the regions K1 and K2 to facilitate the formation of a contact.

The invention has been described with reference to preferred embodiments. It is obvious for the skilled person that changes and modifications can be made to the invention without leaving the scope of the subsequent claims.

LIST OF REFERENCE SIGNS

10 coating apparatus

12 substrate

12 a surface of the substrate

14 particle reservoir

15 particle reservoir

16 particle reservoir

18 supply control device

20 electric arc

22 plasma jet

23 second plasma jet

24 transport pipe

25 second transport pipe

26 coating head

26A exit side

26E feed side

27 second coating head

27E feed side

28 controller

30 process gas

32 process gas

33 process gas

34 first particle supply unit

36 second particle supply unit

37 third particle supply unit

38 process gas control unit

40 particle supply unit

42 process gas control unit

44 process gas control unit

46 first supply location

48 second supply location

50 first layer

52 second layer

53 third layer

54 gradient layer

55 compound layer

56 plasma process gas

58 power supply

60 plasma chamber

62 electrode

64 electrode

66 first injector

68 second injector

70 third injector

72 insulating layer

74 conductive layer

A, B, C, D, E, F segments of a layer

r, s, t particles

S1 layer material

S2 layer material

U transition region

K1 region

K2 region

K3 region 

What is claimed is:
 1. A coating apparatus for coating a substrate, comprising: a plasma generator for generating a plasma jet; which exits from a coating head of the plasma generator from which the plasma jet exits; a first particle reservoir connected with a transport pipe for supplying particles stored in the first particle reservoir to the plasma jet: at least a second particle reservoir is provided and configured to supply particles from the second reservoir via the transport pipe to the plasma jet in a particle mixture with the particles from the first particle reservoir; and a supply control device for setting an amount of the particles fed from the first particle reservoir into the transport pipe relative to the amount of the particles fed from the second particle reservoir into the transport pipe.
 2. Coating apparatus according to claim 1, wherein the supply control device is configured in such a way that a composition of a particle mixture is changeable in a time-dependent manner.
 3. Coating apparatus of claim 2, wherein a controller is provided for controlling the amount of particle mixture supplied to the plasma jet.
 4. Coating apparatus of claim 3, wherein the controller is a switch for allowing and/or interrupting the supply of the particle mixture to the plasma jet.
 5. Coating apparatus of claim 1 wherein a plurality of particle reservoirs is provided.
 6. Coating apparatus of claim 1, wherein at least one separate process gas is provided to be mixed with the particles from one of the particle reservoirs in order to form a fluidized powder.
 7. Coating apparatus of claim 6, wherein a separate process gas is provided for each particle reservoir.
 8. Coating apparatus of claim 6, wherein the particle reservoir and the corresponding process gas form a particle supply unit, the particle supply unit having a process gas control unit for controlling a mixing relation between particles and process gas.
 9. Coating apparatus of claim 1, wherein the coating apparatus has at least a second coating head and at least a further particle supply unit corresponding to the second coating head, the further particle supply unit having a particle reservoir, a corresponding process gas and a process gas control unit.
 10. Coating apparatus of claim 9, wherein the coating apparatus has a plurality of coating heads and corresponding particle supply units.
 11. Method for coating a substrate comprising the following steps: generating a plasma jet with a plasma generator having at least one coating head from the plasma jet exits; feeding particles from at least a first particle reservoir and from at least a second particle reservoir via a transport pipe to a supply control device in which they are mixed; supplying a particle mixture of particles from the first particle reservoir and of particles from the second particle reservoir from the supply control device to the plasma jet via the transport pipe; and directing the plasma jet together with the particle mixture onto a surface of the substrate in order to form the coating.
 12. Method for coating a substrate according to claim 11, wherein the particles from the first particle reservoir are fluidized with a first process gas and the particles from the second particle reservoir are fluidized with a second process gas.
 13. Method for coating a substrate according to claim 11, wherein the fraction of particles from the first particle reservoir in the particle mixture is set between 10% and 90%, and the fraction of particles from the second particle reservoir in the particle mixture is set between 10% and 90%.
 14. Method for coating a substrate according to claim 11, wherein the ratio between particles from the first particle reservoir and particles from the second particle reservoir is changed during the coating of the substrate.
 15. Method for coating a substrate comprising the following steps: generating a plasma jet with a plasma generator of a coating apparatus, the plasma jet exiting from a coating head; supplying particles from a first particle reservoir a transport pipe to the plasma jet at a first supply location; and supplying particles from a second particle reservoir to the plasma jet at a second supply location, wherein the position of the first supply location and the position of the second supply location relative to the plasma jet are chosen in such a way that on the substrate a first layer with particles from the first particle reservoir and a second layer with particles from the second particle reservoir or a gradient layer or a compound layer is formed.
 16. Method for coating a substrate according to claim 15, wherein the second layer or the gradient layer or the compound layer is covered with a further layer, wherein particles from a third particle reservoir are fed into a second transport pipe, supplied to a second plasma jet of a second coating head, and applied onto the second layer of particles from the second particle reservoir or onto the gradient layer or onto the compound layer.
 17. A coating apparatus for coating a substrate, comprising: at least a first plasma generator and at least a second plasma generator each of which generating a plasma jet; a coating head of the first plasma generator from which the plasma jet exits and a coating head of the second plasma generator from which the plasma jet exits; a first particle reservoir connected with a transport pipe for supplying particles stored in the first particle reservoir to the plasma jet of the first plasma generator; at least a second particle reservoir for supplying particles from the second reservoir via the transport pipe to the plasma jet of the first plasma generator in a particle mixture with the particles from the first particle reservoir; at least a third particle reservoir connected with a transport pipe for supplying particles stored in the third particle reservoir to the plasma jet of the second plasma generator; a supply control device for setting an amount of the particles fed from the first particle reservoir into the transport pipe to a first injector relative to the amount of the particles fed from the second particle reservoir into the transport pipe to a second injector, wherein the first injector and the second injector are arranged in relation to the plasma jet of the first plasma generator; and a further supply control device for setting an amount of the particles fed from the third particle reservoir into the transport pipe to a third injector arranged in relation to the plasma jet of the second plasma generator. 